...... blower speed controller .....
thats where i'm putting my money, it probably behind the glove compartment in the plenum. the workshop manual i have is for a 96, but the interior and the HVAC should all be the same.....but since Jdot has the correvt year i think i'll let hime give you some ideas first
as for your milage, when is the last time you changed your air filter/fuel filter. maybe try using a drop in K&N style filter....muscle mustangs and fast fords recently had and article where they put one one those in a SC on a dyno and saw in increase in hp and max boost. might give you some mpg too. also try cleaning your maf with some electrical contact cleaner.
i just blew the head gaskets in my 3.8 so i had everything apart so i just changed everything and cleaned all the intake ports, the heads, new plugs, took the injestors off the rail and cleaned them all. now not only does mt 3.8 with 175k miles on it run like its brand new, i also get great gas milage